During February 1943, Ultra is used by the Allied to guide the convoys around U-boat concentrations. The German B-service on the other hand, is able to decode some or most of the messages concerning convoy routes.

So when the B-service discovers the route of ON-166 and the BdU deploys the patrol lines Ritter and Neptun accordingly, the convoy is re-routed to the South.

When the re-routing is discovered, the gruppe Knappen is formed on the south-east of the Ritter patrol line. One of these boats, the U-604 makes finally contact on mid-day of the 20th.

21 FebruaryU-604 is driven off by the Spencer before other boats arrive, but next day many boats round up around the convoy. U-603 and U-332 sink together a straggler. Aggressive patrolling from the escorts in co-operation with the air escorts prevents any daylight attacks. U-623 is sunk by a 120 Sqdn Liberator, U-91 is damaged by a Catalina and U-332, U-454 and U-753 are driven off. The Polish destroyer Burza, coming from ONS-167 reinforces the escort.

Night of the 21th FebruaryU-91 attacks and torpedoes 2 ships. One ship cannot be salvaged and is finished off by the Burza, the other one tries to escape, escorted by the Dauphin, but has finally to be scuttled by the latter. An attack of U-753 and U-92 on one of this ships fails. U-529 goes missing around this time.

22 February
During day, U-603, U-606 and U-628 keep contact and bring up U-92, U-186, U-223, U-358 and u-753. The escort keeps the boats at bay, but at night U-753 and U-606 attack simultaneously the convoy. U-753 is repulsed by the Trillium and Rosthern, but U-606 torpedoes 3 ships, 2 of which sink and the third is finished off by the U-303.

U-606 is sunk in a counterattack by the Chilliwack, Burza and Campbell. The Campbell is damaged when ramming U-606 and must leave for Iceland. The Burza and Dianthus are short on fuel and leave for Newfoundland. The rescue ship falls behind when looking for survivors and runs into the U-604. U-604 fires a spread of four torpedoes and scores 2 hits on the small ship which sinks immediately.

23 February
The following attacks are facilitated because the escort ships have to do the rescue work and deplete that way the escort screen. In the morning U-628 and U-186 attack together. The former torpedoes 2 ships who are later finished off by U-223 and U-603 and the latter sinks 2 ships.

U-628 and U-707 keep contact with the convoy on the 23th and bring up U-92, U-358, U-468, U-600, U-621 and U-653. The convoy however manages to shake all boats off by a sharp change of course and there are no attacks during night. Only U-707 sinks a straggler and U-653 damages one. Towards morning U-600 damages one ship in the convoy and U-628 sinks it.

24 FebruaryU-603 and U-628 are the contact keepers. They bring up U-600, U-604 and U-621 for the night attack. Air escort from Newfoundland interferes with the build-up, damaging U-604. U-621 has a miss on the Spencer. The British destroyers Witherington and Mansfield reinforce the escort. No attacks are made in the night but in the morning U-92, U-600 and U-628 attack at the same time. U-628 sinks one ship but the other 2 have only misses at the Spencer.

25 FebruaryU-468 keeps contact but only U-600 and U-621 try to come up. They cannot make contact before night and in the morning of the 26th the operation is broken off.